Injection devices for setting and dispensing a single or multiple doses of a liquid medicament are as such well-known in the art. Generally, such devices have substantially a similar purpose as that of an ordinary syringe.
Injection devices, in particular pen-type injectors have to meet a number of user-specific requirements. For instance, with patient's suffering chronic diseases, such like diabetes, the patient may be physically infirm and may also have impaired vision. Suitable injection devices especially intended for home medication therefore need to be robust in construction and should be easy to use. Furthermore, manipulation and general handling of the device and its components should be intelligible and easy understandable. Moreover, a dose setting as well as a dose dispensing procedure must be easy to operate and has to be unambiguous.
Typically, such devices comprise a housing or a particular cartridge holder, adapted to receive a cartridge at least partially filled with the medicament to be dispensed. The device further comprises a drive mechanism, usually having a displaceable piston rod which is adapted to operably engage with a piston of the cartridge. By means of the drive mechanism and its piston rod, the piston of the cartridge is displaceable in a distal or dispensing direction and may therefore expel a predefined amount of the medicament via a piercing assembly, which is to be releasably coupled with a distal end section of the housing of the injection device.
The medicament to be dispensed by the injection device is provided and contained in a multi-dose cartridge. Such cartridges typically comprise a vitreous barrel sealed in distal direction by means of a pierceable seal and being further sealed in proximal direction by the piston. With reusable injection devices an empty cartridge is replaceable by a new one. In contrast to that, injection devices of disposable type are to be entirely discarded when the medicament in the cartridge has been completely dispensed or used-up.
Reusable devices typically comprise an access opening to remove an empty cartridge and to insert a filled or new cartridge into the device instead. Some injection devices comprise a multi-component housing, e.g. a proximally-located body that is releasably engaged with a distal cartridge holder. While the body accommodates and contains the mechanical components of a drive mechanism the cartridge holder at least partially accommodates the replaceable cartridge and keeps the cartridge in a well-defined position in regard to the drive mechanism, in particular in regard to the piston rod. A distal end of the cartridge holder or of the housing of the injection device is releasably connectable with a piercing assembly, typically comprising a double-tipped injection needle that is configured and designed to pierce a distal seal of the cartridge in order to obtain access to the interior thereof for expelling of a well-defined amount of the medicament from the cartridge through the distally-directed advancing motion of the piston rod.
Many injection devices, of e.g. pen-injector type, provide multiple and frequent dispensing and injection of the liquid medicament. For this, the piston rod of the drive mechanisms of such devices advances in discrete steps in a distal direction during repeated dose dispensing procedures until an end-of-content configuration has been reached, in which the piston of the cartridge reaches a distal stop or end configuration. Since the piston rod of the drive mechanism has been correspondingly and successively displaced in distal direction also the piston rod is located in a distal end-of-content position when an empty cartridge is to be replaced by a new or filled one.
For reusable injection devices it is therefore required that the drive mechanism provides a reset mechanism or reset function by way of which at least the piston rod is displaceable into an initial configuration, i.e. a zero dose configuration or zero dose position.